Transcranial magnetic stimulation (TMS) is a noninvasive technique used to apply brief magnetic pulses to the brain, or to other human organs, and to thereby activate neuronal structures. The pulses are administered by passing high currents by a stimulator through an electromagnetic coil externally placed upon the patient (for example, placed on the scalp for brain treatment), inducing electrical currents in the underlying tissue, thereby producing a localized axonal depolarization. This technique has become a major tool in central nervous system research, as well as a potentially promising treatment option for various neurobehavioral and neurological disorders.
Most known TMS coils stimulate superficial brain regions in the brain cortex, but the rate of decay of the induced magnetic and electric field as a function of distance from the coil is high. Hence the efficacy of affecting deeper neuronal structures is low. Stimulating deeper neuronal structures may be feasible if the intensity of the induced field is greatly increased. Yet operation at such increased intensity may increase the risk for seizures and for physiological damage to the tissue.
A method for deep brain TMS with minimal stimulation of superficial regions is disclosed in U.S. Pat. No. 7,407,478, wherein deep brain stimulation is made possible while minimizing side effects. The device described therein includes a base and an extension portion, the base having individual windings for individual paths of current flow, and the extension portion designed so as to minimize unwanted stimulation of other regions of the brain.
However, there is a need for more specifically designed coils, which can target particular areas of the brain including deep neuronal structures with minimal effect on other brain regions. Specifically there is a need for coils which can target unilateral brain regions in a certain hemisphere with minimal effect on the contralateral hemisphere. Examples of specific brain regions that may be desired to be stimulated are the right or left insular cortex. Other examples may include right or left medial and lateral prefrontal cortex (PFC), the Broca's area or it's contralateral homologue, the Wernicke's area or its contralateral homologue, the entorhinal cortex, temporal cortex regions such as the fusiform face area (FFA) and the superior temporal sulcus (STS). The coils must induce the desired distribution of the electric field in the brain, and simultaneously induce electric field intensity in the relevant brain tissue which will be feasible for neuronal stimulation with available TMS stimulators for most of the population. The stimulation intensity is routinely calibrated individually for each subject based on his motor threshold. Hence the coil efficiency must guarantee that the motor threshold and stimulation intensity for most of the relevant population is within an acceptable range with respect to available stimulators power outputs. The coils design must be efficient with respect to energy consumption, coil heating rate, compact size and ease of operation.